What are the PCB Design Rules?

2017-03-27 14:27:15
viya
94

Board Size and thickness

PCB manufacturer needs to set a maximum size for all the PCB boards. As well as their panel size limit. In order to save more space and reduce the costs in mass production, people always use more boards on a single panel.

Board thickness also needs to be specified. A standard thickness and type of board is FR4 .062″ and .010″, .020″, .031″, and .092″.

Width and Spacing

The parameters are always specified as “x/y rules” where x is for the minimum of trace width and y is for the minimum trace spacing. As an instance, “8/10 rules” would indicate 8 mil minimum trace width and 10 mil for the minimum trace spacing. There is a requirement on the minimum spacing between traces and minimum width of trace when producing a PCB. There is some chance trace will open if a trace is made smaller than this minimum width when manufactured. There is also some chance they will short if two traces are closer together than the minimum spacing when manufactured.

In the modern process, the rules of x/y is 8/8, but the small values as 2/2 is also available. Those beginner developers can use 12/12 rules, and larger value can make work consistently. Nevertheless, please remember that the board must be soldered and a trace within 8 mils as 8/8 rules. A pad is easier to short, use one with greater spacing when hand soldered instead. Like 10/10 rules is easier to solder if you have such spacing in your designed board.

PCB Thickness

The 63 mil PCB thickness appears often as the thickness specification for PCBs. The question often asked is why is this thickness specified and is it an industry standard? This is one of those topics that warrants looking at the history of PCBs as they have evolved from simple single-sided boards to dozens of layers. After this review it will be seen that this is one of those specifications not unlike the often told story of how American railroad rails came to be 4’ 8.5” apart.

The manufacturers process four layer PCB boards with final thickness of 0.020″, 0.031″, 0.040″, 0.047″, 0.062″, 0.093″ and 0.125″.Six layer boards are produced in thickness of 0.031″, 0.040″, 0.047″, 0.062″, 0.093″ and 0.125″ with the same inner layer foil options. Both eight and ten layer boards are available in finished thickness of 0.062″, 0.093″ and 0.125″.

Combating Heating Issues

Ever have your circuit performance degraded or even your board damaged because of heat issues? This problem afflicts many designers when heat dissipation isn’t taken into consideration. Here’s some guidelines to keep in mind to help combat heating issues:

Identifying Troublesome Components

The first step is to start considering which components will dissipate the most heat on your board. This can be accomplished by first finding the “Thermal Resistance” ratings in your component’s datasheet, and then following the recommended guidelines to divert the heat being produced. Of course, heatsinks and cooling fans can be added to keep component temperatures down, and also remember to keep critical components away from any high heat sources.

Adding Thermal Reliefs

Adding thermal reliefs can be incredibly useful to produce a manufacturable board and they are critical for the wave soldering application on high copper content assemblies and multilayer boards. Because it can be difficult to maintain process temperatures, it’s always recommended to utilize thermal reliefs on through-hole components to make the soldering process as easy as possible by slowing the rate of heat sinking through the component plates.

As a general guideline, always use a thermal relief pattern for any via or hole that is connected to a ground or power plane. In addition to thermal reliefs, you can also add teardrops where traces join pads to provide additional copper/metal support. This will help to reduce mechanical stress and thermal stress.

Fine-Tuning Your Component Placement

Orientation. Be sure to orient similar components in the same direction as this will help with an efficient and error-free soldering process.

Placement. Avoid placing components on the solder side of a board that would rest behind plated through-hole components.

Organization. It’s recommended to place all your Surface Mount (SMT) components on the same side of your board, and all through-hole (TH) components on the top side of your board to minimize the number of assembly steps.

By following this small list of recommendations, you’ll be well on your way towards designing a functional and manufacturable board in no time.